Conclusion

The Canon EF 40mm f/2.8 STM is an appealingly simple, compact and capable lens that's equally at home on an APS-C or full-frame DSLR camera. It's also very cheap, making it a worthy addition to any EOS user's camera bag.

Despite it's mostly plastic construction and affordable price, the Canon EF 40mm f/2.8 STM build quality is reassuringly good, certainly better than some of Canon's 50mm primes. It's a great match for either a full-frame Canon DSLR like the EOS 6D or an APS-C body like the 700D that we tested it with, where the focal length changes to 64mm.

Auto-focusing is perhaps this lens' Achilles heel. It's quiet but not especially quick, especially in Live View mode. For video shooters, though, the STM motor is appealingly smooth and quiet, although not fast enough to keep up with fast-moving subjects. The ability to instantly manually override the AF system via the focus ring if required is a nice touch, and we didn't really notice or mind that the focusing ring is electronically-driven.

Image quality is generally very good. Chromatic aberrations are well controlled, geometric distortion is impressively low, and the Super Spectra coatings successfully prevent contrast loss attributable to flare. The only real optical issues are some obvious corner shading at the maximum aperture on a full-frame body, and a lack of critical edge sharpness between f/2-2.8 and f/16-f/22. Still, not bad going for a £200/$200 lens.

In summary we can heartily recommend the Canon EF 40mm f/2.8 STM as a ultra-lightweight, extremely portable standard/short-telephoto prime lens for all Canon DSLR owners, at a price that won't break the bank.